1. Field of the Invention
The present invention relates generally to an electric power steering in which steering power assist of an electric motor is directly to a vehicle steering system to reduce necessary steering power to be applied by a driver, and more particularly to such a power steering apparatus which is capable of effectively decreasing unwanted noises in a vehicle steering system so as to provide a highly improved response to driver's steering operations.
2. Description of the Related Art
Electric power steering apparatuses for use with an automotive vehicle have conventionally been known, which supply to an electric motor a target current generated on basis of operator-applied steering torque sensed by a steering torque sensor and vehicle velocity sensed by a vehicle velocity sensor, and drives the motor in accordance with the thus-obtained target current.
There have also been known electric power steering apparatuses which detect a deviation between a target current and a motor current actually flowing through such a motor and compensate for a phase delay and deviation in the control system via a proportional-plus-integral (PI) control or proportional-plus-integral-derivative (PID) control that performs control using the detected deviation.
FIG. 9 is a schematic representation of the general arrangement of a prior art electric power steering apparatus, and FIG. 10 is a block diagram showing the configuration of the principal part of the electric power steering apparatus shown in FIG. 9.
As shown in FIG. 9, the electric power steering apparatus 1 comprises a steering wheel 2, a steering shaft 3, a hypoid gear 4, a rack-and-pinion mechanism 5 composed of a pinion gear 5A and a toothed rack 5B, tie-rods (only one shown) 6, steerable front wheels (only one shown) 7, an electric motor 8 for generating a steering assist, a steering torque sensor 10 for sensing steering torque applied to the steering wheel 2 by the driver and outputting a steering torque signal T indicative of the sensed torque, a vehicle velocity sensor 11 for sensing vehicle velocity and outputting a vehicle velocity signal V indicative of the sensed velocity, and a control unit 12 for controlling the energization of the motor 8 on the basis of the steering torque signal T and vehicle velocity signal V.
As the steering wheel 2 is operated or turned by the driver, the steering torque sensor 10 operatively connected to the steering shaft 3 detects operator-applied steering torque to output a corresponding steering torque signal T, which will be supplied to the control unit 12.
The rotary motion of the steering wheel 2 is transferred via the steering shaft 3 to the pinion 5A, and the resultant rotation of the pinion 5A is converted into linear movement of the rack 5B, which in turn varies the steering direction of the steerable front wheels 7 via the tie-rods 6.
In the meantime, the vehicle velocity sensor 11 detects vehicle velocity to output a corresponding vehicle velocity signal V, which is supplied to the control unit 12.
On the basis of the steering torque signal T and vehicle velocity signal V, the control unit 12 generates motor voltage V.sub.M for supplying motor current I.sub.M to drive the motor 8. The motor 8 thus driven by the motor current I.sub.M provides a steering assist to the vehicle steering system via the hypoid gears 4 so as to permit substantial lessening of necessary steering power to be applied by the driver.
In FIG. 10, the control unit 12 of the electric power steering apparatus 1 includes a target current generator 13, a subtracter 14, a proportional-plus-integral (PI) control 15, a motor driver 16 and a motor current detector 17.
The target current generator 13 sets a target current I.sub.MS on the basis of the steering torque signal T and vehicle velocity signal V supplied from the sensors 10 and 11, by looking up a steering torque (T) - target current (I.sub.MS) characteristic table (TABLE 1) which is plotted by use of vehicle velocity V as a parameter.
The subtracter 14 calculates a deviation .DELTA.I.sub.M between the target current I.sub.MS and the motor current I.sub.M.
The PI control 15 operates to compensate for the deviation .DELTA.I.sub.M by performing PI control and thus generates a control signal Vo for controlling the energization of the motor driver 16.
The motor driver 16 generates motor voltage V.sub.M on the basis of the control signal Vo (e.g., phase width modulation (PWM) signal) so as to provide motor current I.sub.M to drive the motor 8.
The motor current detector 17 detects the motor current I.sub.M and feeds the detected current I.sub.M back to the minus (-) input of the subtracter 14 (NFB: Negative Feedback).
The PI control 15 comprises proportional and integral elements connected in parallel configuration, and gain G and phase angle .theta. for transfer function F(j.omega.) are represented in a Bode diagram of FIG. 12.
As seen from FIG. 12, in a region where the angular frequency .omega. corresponding to the steering rotation speed is relatively low, the gain G (=20 log G) denoted by solid line can be greatly improved although a phase delay (.theta.=-90.degree.) occurs as denoted by broken line; in another region where the angular frequency .omega. is relatively high, the phase delay can be decreased to a substantial degree although the gain G is low.
However, despite the fact that the prior electric power steering apparatus 1 can enhance the gain G in the low angular frequency region and decrease the phase delay in the high angular frequency region as mentioned, the apparatus 1 has the problem that the PI or PID control is very susceptible to unwanted noise brought in from the outside.
FIGS. 13 and 14 are block diagrams corresponding to the configuration of FIG. 10, showing examples where a noise source is included. More specifically, FIG. 13 is a block diagram showing an example where a noise source is present in the feedback loop of FIG. 10, while FIG. 14 is a block diagram showing another example where a noise source is present on the input side of the feedback loop of FIG. 10.
In FIGS. 13 and 14, reference character I.sub.MS denotes the target current I.sub.MS, I.sub.M the motor current, and I.sub.N and I.sub.NO noise currents. Further, reference character A denotes an equivalent circuit of the PI control 15 and motor 8, whose transfer function can be expressed as: EQU A=K.sub.P * {1+1/(T.sub.I * s)} * 1/R.sub.M [Equation 1]
where s represents a Laplace operator, and L.sub.M &lt;&lt;R.sub.M. Further, in Equation 1, K.sub.P, T.sub.I, R.sub.M and L.sub.M represent a proportional gain, integration time, inter-terminal equivalent resistance of the motor 8 and self-inductance of the motor 8, respectively.
Because the noise currents I.sub.N and I.sub.NO are generally of high frequency and hence the Laplace operator s is far greater than "1" (s&gt;&gt;1), Equation 1 may be simplified into Equation 2, which represents the transfer function only by simple gain factors as follows: EQU A=K.sub.P /R.sub.M [Equation 2]
Assuming that noise current I.sub.N is brought into the feedback loop as illustrated in FIG. 13, the relationship among the motor current I.sub.M, target current I.sub.MS and noise current I.sub.N may be expressed as: EQU I.sub.M =(I.sub.MS -I.sub.N) * A/(1+A) [Equation 3]
As clear from Equation 3, the rate of contribution to the motor current I.sub.M of the noise current I.sub.N equals that of the target current I.sub.MS.
Thus, as the proportional gain A is increased in order to improve a response to the driver's steering operation, the noise components in the motor current I.sub.M increases so that the commercial quality of the steering apparatus may be damaged by increased adverse effects resulting from the noise components. Therefore, it is necessary to decrease the proportional gain A to lower the steering response of the apparatus.
Also, where noise current I.sub.N is brought in the target current I.sub.MS as illustrated in FIG. 14, the contribution to the motor current I.sub.M of the noise current I.sub.N equals that of the target current I.sub.MS as expressed in Equation 4 below. Thus, if the proportional gain A is increased in order to improve the steering response, the noise components in the motor current I.sub.M increases so that the commercial quality may be damaged by increased adverse effects resulting from the noise components, as in the case of FIG. 13. Therefore, it is necessary to decrease the proportional gain A to lower the steering response of the apparatus. EQU I.sub.M =(I.sub.MS +I.sub.NO) * A/(1+A) [Equation 4]